Production of nanomaterials has increased continuously because of the unique physicochemical characteristics and extensive applications of these materials. Current scientific evidence indicates that nanoparticles may be more biologically reactive than larger particles of similar chemical composition. Therefore, it is prudent to take measures to minimize worker exposure. Respirators, including NIOSH-certified N95 and P100 respirators, are recommended when engineering and administrative controls do not adequately prevent exposures to airborne nanomaterials. The assigned protection factor (APF) of a certified respirator takes into account total inward leakage caused by (1) penetration of particles through the filter, and (2) leakage around the face/seal interface region. Preliminary laboratory tests of filtering efficiency using standard test aerosols have been reported in the literature. There are no reported studies on respirator filtration efficiency for actual engineered nanoparticles. The preliminary tests also were limited to the filter penetration characteristics. No reports on face seal leakage are available for nanoparticles. Therefore, there is insufficient information on the performance of NIOSH-certified respirators for respiratory protection of occupational exposure to engineered nanoparticles. The overall objective of the proposed study is to evaluate the performance of N95 and P100 respirators for respiratory protection of engineered nanoparticles. Both the filter penetration and face seal leakage of these respirators will be characterized using engineered nanoparticles. Because many nanoparticles have physical properties different from standard test particles, we also plan to characterize the physical and transport properties of engineered nanoparticles. A theoretical framework to relate the key physical and transport characteristics of these nanoparticles to the filtration process will also be developed. The proposed study will have a positive impact on the research of respirator protection of engineered particles by providing a research approach and tool to use realistic engineered particles in the respirator test, design, and development of samplers, and exposure assessment.